Clemens in U.S. Pat. Nos. 3,842,194, 3,842,217 and 3,909,517 has described a capacitive information disc record comprising a molded plastic disc record having audio and video information in the form of geometric variations in a spiral groove in the disc record surface. These disc records are overcoated with a conductive material, which acts as a first electrode, and then further overcoated with a dielectric layer. A metal-tipped stylus acts as the second electrode of a capacitor and the information signals recorded in the disc record surface are recovered as variations in capacitance between the stylus and the disc record surface. Relative motion is established between the disc record surface and the stylus during information recovery. The recovered information signals are processed to reconstitute the audio and video information.
Capacitive information disc records have also been developed which do not require a grooved surface. With this type of disc record the stylus is maintained in synchronization with an information pattern in the disc record surface by means of electrical signals on either side of the information track rather then physically by means of the groove walls.
Further developments, as described in the copending application of Fox et al., "A Conductive Video Disc," Ser. No. 105,550, filed Dec. 20, 1979, have led to a conductive capacitive information disc record which includes a molding resin such as a vinyl chloride homopolymer or copolymer and a sufficient amount of conductive particles, such as carbon black, so that the disc record can provide capacitance readout. The conductive capacitive information disc record may be employed with either the grooved or nongrooved information disc record system. This development has eliminated the need for coatings of conductive metal and dielectric on the disc record.
The conductivity of the molded disc record is generally directly related to the amount of conductive carbon particles added to the molding composition. However, large quantities of fillers, such as carbon black, decrease the processability of a molding composition and increase the brittleness of the molded article. The amount of conductive carbon particles which must be added for the conductive video disc record must be sufficient to reduce the bulk resistivity to below about 500 ohm-centimeters at about 900 megahertz (MHz), preferably below 100 ohm-centimeters at about 900 MHz and more preferably below about 5 ohm-centimeters at about 900 MHz. Thus, for example, a molding composition containing Ketjenblack EC carbon black particles (a product of Akzo Chemie) in amounts of from about 10 percent to about 20 percent by weight has the desired conductivity and processability.
Whipple et al. in a copending application entitled "Method for Preparing a Molding Composition," Ser. No. 231,287, which was filed concurrently and is incorporated hereby by reference, teaches a method for preparing conductive molding compositions which includes blending molding compositions according to their melt viscosities. The Whipple et al. method may be employed to overcome batch-to-batch carbon black variations. They have found that the melt viscosity can be related to the performance properties of the capacitive information disc record.
In order to reliably mass-produce the conductive plastic capacitive information disc records, it is necessary to overcome batch-to-batch carbon black nonuniformities. These variations affect the signal-to-noise ratio of both the audio and video signals. It would therefore be desirable to develop a method of overcoming batch-to-batch carbon black variability which relates to the properties of the carbon black itself to those of the capacitive information disc record.